Role of RNA

Role of RNA in Protein Synthesis

There are several kinds of RNA, perhaps more than we are aware of. The synthesis of protein involves messenger RNA, transfer RNA, and ribosomal RNA.

	The transfer RNA selects the amino acids.
	The messenger RNA dictates the order in which they are sequenced.
	The ribosome, which contains ribosomal RNA combined with protein, carries out the attaching process.

These ribonucleic acids, like the proteins that they help to make, are themselves made up of a sequence of smaller units called nucleotides, and it is the sequence of these nucleotides that determines the sequence of the amino acids and the protein that is being assembled by the ribosome. We will look soon at these individual nucleotide units, how they are attached to one another to make the nucleic acids, and the smaller molecules from which they themselves are made. (That will be in the Nucleic Acid Structure section.)

The sequence of nucleotides in messenger RNA determines how the transfer RNA will line up the amino acids and thus determines the amino acid sequence of the proteins, which is the primary structure of the protein, which in turn determines the secondary and tertiary and,if appropriate, the quaternary structure of the protein, which in turn determines the function of that particular protein.

Codons

A group of three RNA nucleotides makes up a codon and each codon specifies a particular amino acid. Thus a sequence of codons in RNA specifies a sequence of amino acids in protein. There are four kinds of RNA nucleotides and consequently there are 64 different codons (4x4x4). Since there are only 20 amino acids in protein, there is some repetition. Some amino acids are associated with only one codon and others are associated with as many as six codons. Three codons appear to be reserved for starting and stopping sequences. (A table of codon-protein links is shown in Example 15 of your workbook and we will make use of that in the section on Translation.)

The sequence of the nucleotides in the messenger RNA is itself determined by still another chemical. It is, in effect, copied from another nucleic acid called deoxyribonucleic acid or DNA, for short. (We will soon spend quite a bit of time looking at how that copying process takes place in the section on Transcription.)

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